1. Field of the Invention
The present invention relates to a twin shaft Z type propelling means for tug boats, and more particularly to such propelling means which can provide propeller revolution speed slower than the speed normally obtained at minumum revolution speed of the main engine, through slipping of clutch means, thereby improving the maneuverability of the tug boat.
2. Description of the Prior Art
As is well known to these skilled in the art, tug boats must perform various specific functions such as standing, performing front, lateral or slant movements, and going dead-slow, ahead or astern, all while working in limited port areas.
Those functions have been performed, in conventional tug boats having so called Z type propelling means with twin shafts, by controlling the direction of thrust of the two shafts. Namely, for the dead-slow going, the propellers must be so orientated as to diverge as shown in FIG. 1 so as to relieve part of their thrust outwardly, since the dead-slow going is hard to effect with the direction of thrust coinciding with the longitudinal direction of the hull.
Referring to FIG. 1, reference numeral 1 designates Z type propelling means consisting of two propellers, each of which is adapted to generate thrust as represented by vectors F.sub.1. The resultant thrust of those two conponents F.sub.1 is represented by a vector F. Thus, where the angle .theta. at which the shafts are inclined with respect to the longitudinal axis of the hull, the resultant vector F is given by the following equation. EQU F = 2F.sub.1 cos .theta.
It will be understood, when the tug boat must be maneuvered at a speed lower than as could be obtained by vectors F.sub.1 which correspond to the minimum speed of the main engine, the angle .theta. is adjusted to provide fine adjustment of the resultant thrust F.
This way of controlling the thrust is, however, inconvenient in that the violent flow of water orientated laterally of the tug boat sweeps away small boats thereabout, or makes those boats crash into each other or cut fishery nets.